Electrical inductors



May 29, 1962 D. F. CHAPMAN ELECTRICAL INDUCTORS Filed Dec. 30, 1959/A/l/E/VTO Q DOA/A LD F CHAPMAN Unite States atent 3,037,176 ELECTRICALINDUCTORS Donald Frederick Chapman, London, England, assignor to ThornElectrical Industries Limited, London, England, a company of GreatBritain Filed Dec. 30, 1959, Ser. No. 862,978 5 Claims. (Cl. 33683) Thepresent invention relates to electrical inductors.

One use of an electrical inductor is as a choke in circuit with agaseous discharge device, such as, for example, a fluorescent lamp,supplied by alternating current.

It is often required to mount such an inductor in a shallow channelwhich forms parts of a fluorescent lighting fitting. It is also requiredthat the inductor be substantially silent and that stray magnetic fieldwhich could cause vibration of adjacent steel or iron parts of thelighting fitting is not excessive.

It is well known that an inductor or choke having a magnetic core mustbe provided with an air-gap in the magnetic circuit in order to preventflux saturation of the magnetic circuit. Considerable leakage flux ispresent in the vicinity of the air-gap and precautions may be necessaryto shield the inductor so as to prevent this flux leakage and therebyprevent the stray alternating magnetic field from causing noise andvibration in adjacent steel or iron parts which may be located near theinductor.

The stray field will pulsate at 50 cycles per second on ordinary A.C.mains and may cause appreciable hum and noise to be generated by thelighting fitting. Such noise is undesirable.

In one known form of inductor construction the inductor is shielded withthick iron castings in the form of plates which are clamped around theinductor. These castings effectively form a magnetic screen for theinductor but add considerably to the cost and weight of the inductor.

It is an object of the present invention to provide an improved inductorsuitable for use as a choke in a fluorescent lighting fitting andhaving, in operation, stray magnetic fields of such small value as notto cause appreciable noise to be produced in neighbouring parts of thefitting.

According to the present invention there is provided an inductorcomprising a winding and a magnetic circuit of laminated ferro-magneticmaterial, the magnetic circuit 'being in the form of an inner corearound which the winding is disposed and an outer shell co-operatingwith the inner core and comprising two laminated U-shaped yokes fittedone into the other, the limbs of each yoke bridging the gaps between theside edges of the limbs of the other and the transverse portion of eachyoke bridging the gap between the ends of the limbs of the other yoke,the magnetic circuit including a series gap in the inner core or betweenthe inner core and the outer shell or both.

An inductor embodying the invention will be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a somewhat diagrammatic exploded perspective view of theinductor, and

FIG. 2 is a side elevation of the assembled inductor.

The inductor includes a winding of enamelled copper wire on an innermagnetic core. This inner core comprises two stacks 11, 12 ofstrip-shaped laminations of silicon-iron, the two stacks being separatedby a gap 13 filled with a non-magnetic cement. A suitable cement is oneof the well known epoxy resin compounds. The laminations are coated inthe usual way with an insulating material, such as, for example,varnish, in order to reduce eddy-current losses.

The magnetic circuit of the inductor is completed by an outer shellcompletely surrounding the winding and inner core. This shell comprisestwo elongated, U-shaped, laminated yokes 14, 15 so fitted into oneanother as to form a closed, elongated, box-shaped structure 16 (seeFIG. 2). The limbs of the elongated U-shaped yokes 14, 15 constitute thefour adjacent side faces respectively of the outer shell and theirtransverse portions constitute the two end faces. This box-likeconstruction is achieved by fitting the U-shaped yokes together in themanner shown in FIG. 2, that is the limbs of each yoke bridge the gapsbetween the side edges of the limbs of the other yoke and the transverseportion of each yoke bridges the gap between the ends of the limbs ofthe other yoke.

Thus the outer shell serves to screen magnetically the gap in the innercore and to reduce flux leakage from the Although the U-shaped yokes 14,15 need not necessarily be elongated, the form of construction describedis particularly suitable for an inductor which is long and slim and istherefore of a convenient shape for mounting in assemblies such asfluorescent lighting fittings.

In order to reduce still further the flux leakage which would occur inthe case where one central gap is provided in the inner core, twofurther gaps 17, 18 may be provided between the ends of the inner coreand the end faces of the outer shell. These gaps may also be filled witha non-magnetic cement. It has been found that provision in this Way ofthree relatively small gaps considerably reduces flux leakage ascompared with that occurring in an inductor assembly having onerelatively large gap. All three gaps are, of course, completely screenedby the outer shell.

Measurements obtained on an watt fluorescent lamp choke constructed asdescribed and placed inside a sheetsteel channel showed that there wasno detectable increase of vibration or noise due to stray magnetic fieldeifects.

This 80 watt choke, suitable for a 230/2/l0 volts 50,

cycles A.C. supply, was constructed as follows:

The two yokes 14, 15 forming the outer shell were each made from sixstrips of 19" x 1% x .02" dynamo iron bent at the centre to give a widthof 1% between the limbs of the U-shaped member thereby produced. Thecentre core was made from two deep stacks 11, 12 of strips of 4 x x .02"dynamo iron. The coil 10 comprised 1900 turns of 25 S.W.G. enamelledcopper wire layer wound in 6 layers round the core. The gap 13 betweenthe two halves of the inner core and the gaps 17, 18 between the ends ofthe inner core and the end faces of the outer yokes were each 0.05 inchwide.

I claim:

1. An electrical inductor comprising an inner core of laminatedferromagnetic material having a series gap therein filled with anon-magnetic material, a winding disposed around said inner core, and anouter shell of laminated ferromagnetic material, said outer shellcomprising two laminated, U-shaped yokes fitted together to form ahollow enclosed shell surrounding said winding and said inner core.

2. An electrical inductor according to claim 1 wherein gaps filled witha non-magnetic material are provided between the extremities of saidinner core and the adjacent end-faces of said outer shell.

3. An electrical inductor comprising an inner core of laminatedferromagnetic material, a winding disposed around said inner core, and:an outer shell of laminated ferromagnetic material surrounding saidwinding and said inner core, said outer shell comprising two laminated,U- shaped yokes fitted together to form a hollow enclosed shell, gapsfilled with a non-magnetic material being provided between theextremities of said inner core and the adjacent end-faces of said outershell.

4. An electrical inductor comprising an elongated inner core oflaminated ferromagnetic material having a cen- Wally-disposed series gaptherein filled with a non-magnetic material, a Winding disposed aroundsaid inner core, and an elongated outer shell surrounding said windingand said inner core, said outer shell comprising two laminated,elongated U-shaped yokes of laminated ferromagnetic m aterial fitted oneinto the other, the limbs of each yoke bridging the gaps between theside edges of the limbs of the other yoke and the transverse portion ofeach yoke bridging the gap between the ends of the limbs of the otheryoke, gaps filled with a non-magnetic material being provided betweenthe extremities of said inner core and said transverse portions of saidyokes.

References Cited in the file of this patent UNITED STATES PATENTS2,948,871 Craige Aug. 9, 1960 FOREIGN PATENTS 647,995 Great Britain Dec.28, 1950

